The present invention relates to generation of power control bits in a communication system and, more particularly, to a method and apparatus for forward link power control bit generation in a spread-spectrum communication system such as a code division multiple access (CDMA) communication system.
Communication systems are known to employ power control methods that control the transmission energy of remote or mobile units. One such communication system employing power control is a spread-spectrum communication system. Power control in a spread-spectrum system serves two primary functions. First, because each remote unit's signal in a spread-spectrum system is typically transmitted at the same frequency, a majority of the noise associated with a received signal can be attributed to other remote units' transmissions. The magnitude of noise is directly related to the sum of the received signal power of each of the other remote units' transmissions. Thus, it is beneficial to control power such that a remote unit transmits at the lowest power level possible. Secondly, it is desirable to dynamically adjust the power of all remote units in such a way that their transmissions are received by a base station with approximately the same power level. To accomplish this, it is necessary for the closest transmitters to reduce their power as compared to the power of the furthest transmitters.
In some spread spectrum systems such as CDMA systems, a traffic channel forward link signal includes reverse link power control bits in the data stream that allow a base station to individually control the reverse link transmission power of a remote unit (i.e., mobile receiver unit) in its coverage area. After extracting a reverse link power control bit, the mobile unit responds to the power control command by adjusting its transmission power in the reverse link. This power control method accommodates changing channel conditions, such as fading or blocking, while retaining spectral efficiency and minimizing interference within the system. Moreover, the mobile unit can also be utilized to aid the base station in the control of the power on the forward link fundamental channel by transmitting forward link power control bits to the base station by puncturing the bits onto a pilot channel of the reverse link. This technique is particularly utilized in fast forward power control.
Within the context of the CDMA2000 standard, fast forward power control is utilized as a technique to mitigate the effects of fast fading. In this technique, an estimate of the signal to interference plus noise ratio (SINR) determined at the remote unit is compared to a threshold (Γ) in order to generate the power control bits (PCB's) for forward power control. Dependent on whether the SINR is greater or smaller than the threshold Γ, the PCB is typically set to either a binary “1” or “0” (e.g., “0” indicating increase power and “1” indicating decrease power) and transmitted to the base station by puncturing on the reverse link pilot channel. The threshold Γ is determined by outer loop control and typically set every 20 msec. Additionally, the reverse link power control bits punctured on the fundamental forward link channel are sent at a known power level and rate relative to the full rate of the forward fundamental channel.
A problem with the type of fast forward power control described above is that for the forward link control a reliable estimate of the SINR must be found by the mobile receiver without excessively utilizing the limited processing resources of the mobile receiver. Effective and efficient power control is vital to system performance. Thus, there is a need for rapid and accurate detection of information used for spread spectrum power control, such as characteristics of the traffic channel forward link signals (e.g., the SINR) without significantly increasing processing requirements.